Clouds Bases

  • @TZ To be consistent with ATC vocabulary, the current display of Cloud Bases (because they are in AGL) should be renamed Ceilings. I would also suggest adding a separate display called Cloud Bases which would be in MSL.
    I have few complaints about the fine Windy product... but the above misuse of AGL for MSL, also occurs with Freezing Level, which currently is AGL and be much more useful if displayed in MSL. When we pick an altitude to fly, we want to know forecasted Freezing Level in MSL.
    Thanks for a fine product.
    Wayne, pilot

  • Moderator

    Yep, good point, but I think It would be simplier to use the same feature if you're a pilot or not.

    First, it might be a good idea to explain what are height and altitude to other users => height has the ground level for reference (AGL-above ground level). Altitude has the main sea level for reference (MSL)

    There are in my opinion two options regarding aeronautical weather data :

    -Forecaster point of view: Using charts, he gives the ALTITUDE of clouds (from MSL - main sea level) and doesn't care how you use the info...

    -Pilots or controllers point of view: They don't want you to crash and provide the HEIGHT (from AGL - above ground level) of cloud base. So you have an idea of what you're going to see (or not...) while arriving or departing from an airport, whatever the altitude of the airfield.

    Considering this, I think that giving the HEIGHT of cloud base is the best option (AGL), which is already the case. It is also good for non pilot users as it provides a useful and easily understandable info.

    Regarding Freezing level... It's indeed more useful for pilots when using ALTITUDE (MSL) for other technical reasons. And... It's more logical for pilots but also for other users ... For hikers for exemple. They want to know the freezing ALTITUDE and not height, so they can compare it with the ALTITUDE of a mountain for exemple, because what is depicted on topo charts are ALTITUDEs.

    A good solution in my opinion would be to use the words "height" and "altitude"in windy layers names and provide a clear definition of these 2 terms in the help forum section/legend, and obviously use MSL for freezing level and AGL for cloud base.

  • Moderator

    Freezing level, (named Zero Degree Level (DEG0L) by ECMWF with ID=228024),
    is the height of level (counted from 'surface')
    where the temperature passes from positive to negative values.

    Model's "surface" (named Orography(Z) by ECMWF with ID=129)
    over rough terrain may vary a lot compared with actual altitude from MSL.

    It might be a good idea to have a Freezing Level chart using altitude from MSL,
    where FRZ LVL = DEG0L+Z

  • Sailor

    Excellent presentation and analysis of these issues of height and altitude. It would be a good idea to clarify them.
    I am not a pilot, but nevertheless interested to have the Freezing altitude expressed not by the height AGL, but by the altitude above MSL. This would be a useful mean to get a better idea of ​​the rain-snow limit in mountain, which global models are unable to provide given their horizontal and vertical low resolution.

  • Moderator

    ... also, I want to point out that,
    acording to "Database Ref. Manual for ICON"
    ( ), page 33

    for ICON model, the Height of 0 degree Celsius isotherm (short name HZEROLOC)
    has MSL as a reference.

    Thus, the frz. alt. from ECMWF and frz. alt. from ICON, are different!

    Nuremberg's altitude is 314m

  • Moderator

    ahh nice about ICON I didn't check it because my area is only covered by GFS and ECMWF.

    Yeah I think those changes wouldn't be hard to make... But that's a non-developer point of view :)

  • Administrator

    @Gkikas-LGPZ this is important info thank you

  • Sailor

    OK your example in Nuremberg seems to demonstrate that ICON 0°C isotherm is MSL, which is confirmed by the Ref. Manual.

    But how do you explain that this morning in Greece ICON shows on Windy a Freezing « altitude » at 0m in mountains of 1800m alt. approx. and, at close distance, in low land of 100m alt. approx., a Freezing « altitude » at 1300m.


    Switching to ECMWF we see the same figures at same places, but just with a lower resolution map.
    For me both ICON and ECMWF on Windy seem to display a Freezing HEIGHT AGL.

  • Moderator

    It seems that you are right!
    So, I can give you those two explanations:

    1. DWD ICON ref. manual is wrong

    2. Windy uses a different parameter!

    I hope Windy team will investigate that.

  • Moderator

    Please check if Windy uses HZEROLOC parameter for frz. alt.(ICON).
    Read the comments above.

  • Moderator

    You speak German fluently. Isn't it?
    I'd be grateful if you ask DWD if there is a mistake in their reference manual
    (description of HZEROCL, Table 7.5, page 46 on Ref. Manual version 1.2.6).
    Above msl or above surface?

  • Moderator

    I'm at vacation tomorrow, so I write a text for you here and you can send it to dwd

    Sehr geehrte Damen und Herren,
    Ich hätte eine Frage zur Dokumentation des Icon Modell. In der Doku "insert link to dokument " sprechen Sie bei der Beschreibung von HZEROCL, Tabelle 7.5, Seite 46,Version 1.2.6 von MSL oder über Grund Höhe?

    Mit freundlichen Grüßen
    "your name"

  • Moderator


    DWD answered:
    " the freezing level (HZEROCL) is provided in m ASL,
    but it is set to zero (rather than the height of the model topography)
    if the freezing level is at the surface.

    However, I see that this should be changed in our diagnostic
    calculation, as well as the scanning direction (which should be top-down
    rather than bottom-up) in order to detect the uppermost freezing level."

    So, dear idefix37, I think it answers your/our question!

    btw how does AROME handle the subject?

  • Sailor

    @Gkikas-LGPZ said in Clouds Bases:


    I think it answers your/our question!

    btw how does AROME handle the subject?

    Yes, I think... but to clarify the "Freezing altitude" layer on Windy, should we understand that this level is counted as Altitude above sea level, but when this level intersects terrain located at a higher altitude , then this level is measured in relation to the ground itself, as Height above the ground.
    So this layer should be called FREEZING LEVEL, as sometime it is measured as ALTITUDE above sea level (asl), sometime as HEIGHT above ground.

    In Windy, if you check the Freezing level with AROME it switches to ECMWF. So this layer is not available with this model

    In the example below (left map) the Freezing level is at 2400m in West of the Alps and at 0m just above the high summits, as described here above for ICON.
    On same locations, the Freezing altitude from AROME data on Meteociel (Right map) is shown at 2700-2600m even when this altitude intersects mountains at 3000 or 4000m.


    So there are different ways to display the Freezing level as we could try to summarise with this sketch:


    I am more familiar with Freezing altitude asl, which is much more precise for outdoors activity in mountains. The Height above ground is too coarse due to the low altitude resolution of the orographic model integrated in the model itself.

    Anyway, at least, it clarifies the way how this layer must be interpretated...

    Thanks for your investigation


  • @TZ Hi there, I just checked this old post and it seems like Windy cloud base can still be < 50% cloud cover so you never changed over to ECMWF Ceiling? or am I missing something here in New Zealand? Lots of days when cloud base green around 2000m but cloud cover only say 20%.

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